The lichen secondary metabolite usnic acid exists as a (-) and a (+) enantiomer, indicating a alpha or beta projection of the methyl group at position 9b, respectively. (-)-Usnic caused a dose-dependent bleaching of the cotyledonary tissues associated with a decrease of both chlorophylls and carotenoids in treated plants whereas no bleaching was observed with the (+) enantiomer. (-)-Usnic acid inhibited protophorphyrinogen oxidase activity (I50 = 3 microM), but did not lead to protoporphyrin IX accumulation. Bleaching appears to be caused by irreversible inhibition of the enzyme 4-hydroxyphenylpyruvate dioxygenase by (-)-usnic acid (apparent IC50 = 50 nM).